The invention relates to a subscriber line interface circuit for connecting a telephone line to a telephone exchange, the interface circuit comprising a pair of amplifiers which are fed by a d.c. supply source, the outputs of which are connected to the wires of the subscriber's line.
To the telephone line the interface circuit must show an impedance of a certain value, for example, equal to 600 Ohm, it being necessary for the impedance to be balanced relative to ground.
On the other hand, with respect to the direct current supplied to the telephone, in particular subscriber's line, by the supply source, the interface circuit must have a predetermined resistance which, generally, differs from the impedance for the speech signals and which is uniformly distributed over the two terminals of the supply source.
French Patent Application No. 7,731,145 describes a subscriber line interface circuit comprising a pair of amplifiers for feeding the subscriber's line with direct current and control means for these amplifiers for feeding the subscriber's line with an a.c. ringing voltage, without the use of a transformer. In this interface circuit a transformer is used for the transmission of the speech signals.